<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>颜色与不透明度</title>
</head>
<body>
<canvas id="canvas" width="700" height="500" style="border: thin solid black"></canvas>
</body>
<script>
    var canvas = document.getElementById('canvas'),
        context = canvas.getContext('2d');

    context.lineJoin = 'round';
    context.lineWidth = 30;

    context.font = '24px Helvetica';
    context.fillText('点击任何处以擦除', 175, 200);

    context.strokeStyle = 'goldenrod';
    context.fillStyle = 'rgba(0,0,255,0.5)';

    context.strokeRect(75, 100, 200, 200);
    context.fillRect(325, 100, 200, 200);

    context.canvas.onmousedown = function (ev) {
        context.clearRect(0, 0, canvas.width, canvas.height);
    }
</script>
</html>